Aromatic hydrocarbons, particularly benzene, toluene and xylene (BTX), are a by-product formed in the production of gasoline in an oil refinery. The separation of BTX from other hydrocarbons, for example, using a catalytic reformer and an extraction device, results in a BTX mixture containing contaminants, particularly reactive contaminants, such as olefins and diolefins. These contaminants must be removed from the aromatics to obtain the aromatic hydrocarbons in a useful form, particularly for nitration-grade aromatics.
One of the specifications of nitration-grade aromatics is called an Acid Wash Color Test. This involves washing the sample with concentrated sulfuric acid and comparing the resultant color produced with a set of standards. The darker the color, the greater the concentration of impurities, particularly diolefins. This test was introduced in the days when benzene was made from coal tar, by treating it with sulfuric acid. A low Acid Wash Color indicated that the manufacturer had used a sufficiently large quantity of sulfuric acid to remove all the unwanted, highly reactive contaminants.
Olefins are objectionable in aromatic hydrocarbon, e.g., BTX, products for the following reasons: they are reactive and produce undesirable co-products in downstream processing, such as colored materials, such as benzene sulfonates; olefins in xylenes in the production of various xylene derivations, e.g., trimethylxylene occupy sieve capacity thereby adversely affecting performance; and olefins can cause fouling in high temperature equipment, such as a xylene column reboiler.
Another test for detecting the presence of olefins in hydrocarbons, e.g., aromatic hydrocarbon mixtures, is Bromine Index or Bromine Number. Specifically, the Bromine Index is a measure of the milligrams of bromine consumed by 100 grams of sample under given conditions, and Bromine Number is a measure of the grams of bromine consumed by 100 grams of sample. A comparison of the different bromine tests is shown in Table I.
TABLE I ______________________________________ BROMINE INDEX/BROMINE NUMBER METHODS ASTM ASTM ASTM ASTM D 1159 D 1491 D 1492 D 2710 ______________________________________ Analysis: Number Index Index Index Title: Bromine Bromine Bromine Bromine Number of Index of Index of Index of petroleum Aromatic Aromatic Aromatic distillates hydro- hydro- hydro- and commer- carbons by carbons by carbons by cial aliphatic potentio- coulometric electro- olefins by metric titration metric electrometric titration titration titration Definition: grams Br per milligrams milligrams milligrams 100 grams Br per 100 Br per 100 Br per 100 sample grams per grams grams sample sample sample Catalyst None Chloride (HG): Acetate None Reaction 0-5 Ambient Ambient 0-5 Temp. .degree.C.: Titrant 0.5N 0.02N Glacial 0.05N (Bromide/ Acetic Bromate): Methanol KBr, HgAc Titration (CCl.sub.4 / CCl.sub.4 C.sub.2 H.sub.3 Cl.sub.3 Solvent C.sub.2 H.sub.3 Cl.sub.3 (CCl.sub.4 / C.sub.2 H.sub.3 Cl.sub.3) ______________________________________
For example, olefinic compounds are present in hydrogenated, steam-cracked naphtha. The olefin concentration is higher at the end of the hydrogenation processing, whereas at the start of hydrogenation, the Bromine Index of a hydrogenated, steam-cracked naphtha is typically in the range of 100 to 200 (milligrams bromine per 100 grams of sample).
Olefins will always be present in the catalytic reformate, product of a catalytic reformer that is operated under severe conditions (i.e., low pressure). Although the concentration of diolefinic compounds in catalytic reformate are so low as to be hardly traceable by modern laboratory techniques, they are sufficient in quantity to render the Acid Wash Color Test of the aromatic products off-specification. The Bromine Index of catalytic reformate is typically 1000 to 2000 (milligrams bromine per 100 grams of sample), having a Bromine Number of one or two.
The great majority of olefins entering as an extractor feed, together with other reformate products and selective extraction solvents, e.g., triethylene glycol, tetraethylene glycol (TEG) and SULFOLANE (tetrahydrothiophene), are removed from the extractor with raffinate and sent to storage. The raffinate from catalytic reformate feed is typically in the range of 5000 to 8000 milligrams bromine per 100 grams of sample, or a Bromine Number of five to eight. Olefins, and in particular diolefins, are somewhat polar and are, therefore, partly extracted along with the aromatics and leave with the extract product. The quantity of olefins in the hydrocarbon extract is very small in comparison to the olefin content in the raffinate or extractor charge. The Bromine Index of a hydrocarbon extract product from catalytic reformate feed is typically in the range of 50 to 100 milligrams bromine per 100 grams of sample. This would be sufficient to render the Acid Wash Color Test of the product aromatics off-specification. It is, therefore, necessary to treat the extract product derived from extracting raffinate for removal of these olefinic compounds. A comparison of the bromine indices of extractor feed to bromine index of extract and aromatic product is shown in Table II wherein the extract product and aromatic products were derived from catalytic reformates and hydrogenated steam-cracked naphthas.
TABLE II __________________________________________________________________________ BROMINE INDEX VALUES OF TYPICAL SULFOLANE UNIT STREAMS (LABORATORY FRACTIONATIONS UNTREATED EXTRACTS EXTRACTOR BENZENE TOLUENE XYLENE HEAVY REFINER FEED EXTRACT PRODUCT PRODUCT PRODUCT AROMATICS __________________________________________________________________________ Catalytic Reformate Feed (UOP Platforming) U.S. Gulf Coast 79 7 32 85 145 U.S. Gulf Coast 121 46 112 135 320 U.S. Gulf Coast 1700 60 20 50 70 U.S. Gulf Coast 1300 90 20 60 Hydrogenated Steam-Cracked Naphtha Feed Japan 150 20 12 24 17 Japan 200 30 16 46 17 Japan 450 10 30 100 Japan 700 10 50 100 Japan 100 6 6 11 7 90 __________________________________________________________________________
It has been found that there are only two practical methods for improving the Acid Wash Color of the aromatic products resulting from solvent extraction of a reformate product (from catalytic reformation of light hydrocarbons, e.g., C.sub.1 -C.sub.4): hydrotreating the extractor feed; and bentonite clay catalyzing the extract stream to polymerize the olefins for separation of olefins from the aromatic hydrocarbons. Hydrotreating the catalytic reformate saturates the olefins and diolefins. However, hydrotreating is too expensive to be employed in actual practice since it requires a catalyst, a high pressure reactor vessel and other associated equipment. In addition, a source of hydrogen makeup is required which may or may not always be available.
A more attractive and less expensive method is the percolation of the extract product from reformate over an acid-activated clay. The clay partly absorbs the olefinic materials and partly acts as an acidic catalyst to polymerize the mono-olefins into higher boiling polymeric, e.g., dimeric, compounds, which are eliminated in subsequent fractionation steps.
The present invention is directed to a method of manufacturing a bentonite clay-based catalyst, in granule form, by acid binding smaller (fine) clay particles such that the bound clay particles, in the form of acid-bound granules, have sufficient breaking strength and structural integrity to perform as a olefin polymerization catalyst, with substantially increased yield (.apprxeq.100%) due to the capability of recycling all fine clay particles produced in grinding.